Electric shock defibrillation is a proven technique of treatment of the serious and immediately life-threatening condition of ventricular fibrillation (VF). For patients known to be at risk, an implantable defibrillator may be used. Such devices contain an energy source, an electrode lead system in contact in the heart, a sensing system to detect the onset of fibrillation, and a pulse generator for delivering the defibrillation (DF) shock. Often they are combined with a pacemaker function in the same device.
Existing devices are generally designed or programmed to deliver a shock or series of shocks at a fixed interval or intervals following the detection of the fibrillation, unless fibrillation spontaneously terminates on its own first, or until recovery is achieved, as evidenced by the resumption of normal ventricular rhythm. The amount of energy to be delivered in a shock must be carefully chosen. If too small, it may not be successful in terminating the fibrillation. 0n the other hand, the shock must not be too large, from physiological considerations, and also in consideration of the limited energy storage in an implanted device.
It is also known in the treatment of tochyarrhythmia to use an implantable atrial defibrillator to deliver pulses of defibrillating energy to the atria synchronized with sensed R waves of the ventricle. However, in the case of VF, there is not an R wave to synchronize to, so the DF shock must be delivered asynchronously.
It is known that ventricular electrical signals during fibrillation may exhibit a pattern, known as "fine VF," characterized by relatively low signal amplitude and lack of organized features; and they may also exhibit a pattern known as "coarse VF," subjectively characterized by intervals of higher amplitude, which may repeat, separated by fine VF intervals. It has also been suspected that it is easier to defibrillate coarse VF than fine VF. Because of this, previous works have suggested the possibility of timing of DF shocks to features of the VF waveforms as a way to improve DF efficacy. However, it has not been clear from such prior works, which features are important, and how to detect and coordinate to them.
One experimenter retrospectively noted diastolic periods in the monophasic action potential (MAP) tracings, and suggested these periods were more conducive to defibrillation. Another retrospectively observed that some subthreshold defibrillations which were successful had a fixed timing relationship with a bipolar sensing signal in the right ventricle of dogs. However, another study examined spatial coherence in VF on surface of heart using epicardial sensing electrodes, and concluded that coarseness and fineness of VF was mainly due to lead orientation, and not to the degree of organization of electrical activity as measured. Therefore, there appears to be no firm correlation per se recognized in the prior art between DF shock timing and VF features, especially one that may be successfully applied prospectively. One recent study retrospectively examined the correlation between the voltages measured on the surface leads and the energy required to defibrillate dogs instrumented with epicardial patches. Some reduction in energy requirements was found with defibrillation shocks that happened at places where measured voltages were "high."
It is clear that while a number of investigators have pointed to the possibility of using VF waveform features as a guide to delivering DF shocks, there are problems to be solved in the practical and effective prospective detection of VF features, and the determination of which features thereof are significant, in terms of coordination of DF shocks, for maximizing efficacy.